This invention relates generally to trimmed surge resistors for electrical circuits and, more specifically, to the modification of such resistors to achieve precision tolerances.
The development of various ceramic and/or cermet materials made possible many of the recent developments in resistor technology. Such materials are very stable at high temperatures and capable of withstanding temperature and voltage extremes. It is possible to customize these materials to provide resistance values which range from an ohm or less to megaohm values. Such variations in function may be incorporated into the materials during manufacture and/or during the associated firing and/or curing processes. A particular benefit of these materials is that they can be applied to a variety of circuit substrates using well-known thick film techniques such as screen printing.
Neither the materials nor their application lend themselves to an exacting resistance value. As a practical matter, excess material is applied to, for instance, a substrate, then trimmed or removed from the resistor. Removal techniques have become more sophisticated in recent years and now encompass the latest laser technology. Any trim or cut must be made such that the resulting resistance value is less than or equal to the value desired, as additional trimming can only increase the final value. Regardless of the technique, the material removed must provide the desired resistance value.
Various concepts related to the trimming of excess material are provided by way of the prior art, in particular in U.S. Pat. Nos. 5,043,694 and 5,504,470--both of which are incorporated herein by reference in their entirety. The importance of resistor trimming is amplified in the situation where there can occur large electrical surges across a circuit. Survival of the resistor requires a high quality material and thick film. Compositional deficiencies or defects in the application may lead to failure under high surge conditions.
U.S. Pat. No. 4,528,546 discusses resistors in the context of high surge conditions. Incorporated herein by reference in its entirety, this prior art patent illustrates, schematically, a typical resistor component. Electrical terminals can be formed by screen printing a conductive composition on a ceramic substrate. A ceramic and/or cermet material can then be screen printed on the substrate to connect the terminals.
Typically, the initial resistance value of a resistor will be known, calculated based on the number of ohms per square of resistor, where a square is one unit of equal length and width. Decreasing the effective width of a resistor to one-half the original will double the number of squares and simultaneously double the resulting resistance value. Trimming can be conducted with intermittent resistance measurements. However, for precise resistors, resistance is monitored while the trimming proceeds. For instance, when a desired resistance level is reached, the laser trimming device can be disengaged to prevent further removal of resistor material.
U.S. Pat. No. 4,528,546, as referenced above, illustrates a simple plunge cut into the resistor. This method of trimming is quite simple and serves sufficiently to increase resistance value. However, a large voltage gradient, resulting from the redirection of current, will invariably exist across the cut. In addition, it has been shown that the current crowds into a region surrounding the terminus of the cut opposite the end point on the resistor edge. In addition, during a surge condition, the voltage gradient may cause arcing across the cut/trim, arcing of the sort which could also destroy the resistor.
Plunge cuts can be used alone or in combination with a variety of scan cuts. Scan cuts are made parallel to the current flow and alleviate current crowding conditions. The cut/trim is relatively simple and the increase in resistance directly calculated. When used in combination with one or more plunge cuts, current is prevented from flowing into the resulting trimmed portions.
U.S. Pat. No. 5,504,470 describes combining an initial terminal to terminal scan cut with a plurality of plunge cuts, otherwise referred to as a comb cut. The comb cut is described as preventing arcing due to excessive voltage gradients across each plunge. The cuts and/or trimming is described in conjunction with many different prior art resistor devices and a variety of removal methods, all of which are specifically incorporated by reference herein in their entirety. The cuts are straight and easy to incorporate. Because there are no endpoints in the resistive portion of the film.
U.S. Pat. No. 5,043,694, as referenced above, describes a variety of cuts/trims of the prior art, including various u-shaped, square and semi-circular cuts. This patent departs from the prior art in describing a trimming groove having two curved portions linked by a straight portion parallel to a linear resistor edge. The resulting configuration resembles the outline of a paper clip placed along the edge of a resistor, with the curved portions directed toward the electrodes. Various cut/trim configurations are also described inside the outline to further precisely trim or adjust resistance values.
Nonetheless, the prior art has associated with it various problems and deficiencies. With reference to the resistor described in the '470 patent, exacting precision is required in the trimming to eliminate any film on either end of the scan cut or between the plunge and scan cuts, to prevent extreme current crowding and certain circuit failure. This situation is avoided with the resistor described in the '694 patent: curved cuts avoid the residual film problem. However, trimming in this manner is relatively time-consuming. Regardless, with either a scan cut or a curved cut, resistance is, in large part, controlled by the initial cut. Further trimming to increase resistance precisely to a desired value is difficult. There is a need for a trimmed surge resistor and method for its preparation, to provide benefits not otherwise possible.